The RSPCA is continuing to care for 237 seabirds washed ashore following last week's mystery oil spill in the Channel.

Thirty-one birds have had to be put down and another eight have died, the animal charity said on Monday.

The vast majority of the birds – 257 – are guillemots, long-lived birds that spend most of their time out at sea, only coming to shore to breed.

The other 19 are razorbills, another member of the auk family.

Environment Agency scientists are still trying to establish exactly what the oily substance is, while the Maritime and Coastguard Agency is investigating the source of the spill.

Though the number of birds being washed ashore on the south coast of the UK has dropped, the RSPCA and the RSPB fear this could be because the wind over the weekend would have blown other birds affected away from the coast.

Grahame Madge, a spokesman for the RSPB, said: "The winds on Friday were encouraging birds to come into the shore where they could be picked up, but the wind since has been pushing birds more out to sea.

"It is quite likely that there have been many, many more birds affected who have no hope of rescue at the moment."

Madge said birdwatchers on Chesil beach in Dorset had spotted what they believed were many sea birds covered in the pollutant and in difficulty."

On Friday, initial tests revealed the mystery substance as a refined mineral oil. Madge said: "We do hope that the authorities will be able to identify the exact type of pollution and that might give us a clue to where it has come from.

"We have a major pollution incident affecting hundreds if not thousands of birds and we feel it is important that whoever is responsible for this pollution incident is held to account."

The RSPCA, which has been treating the birds at its West Hatch centre near Taunton in Somerset, has put a second centre further east on standby in case more capacity is needed.

Peter Venn, the manager of the West Hatch centre, said: "It is still early days and hard to say how the birds will survive in the long-term. We don't know what this substance is or what it might be doing to the birds."

West Hatch staff first tried to clean the birds with normal soapy water, which was not successful. They had more success cleaning them with margarine.